(1). Field of the invention.
The invention relates to a method of digitizing a color video signal using transform coding.
The invention also relates to an encoding arrangement for putting this method into effect.
(2). Description of the prior art.
Over the years a number of methods have been described for digitizing a PAL or an NTSC color video signal; namely:
(a) pulse code modulation, abbreviated to PCM;
(b) differential pulse code modulation, abbreviated to DPCM;
(c) transform coding, abbreviated to TC.
In PCM the video signal is sampled with a predetermined sampling frequency f.sub.s which is at least equal to the Nyquist sampling frequency. This results in video signal samples which are quantized and encoded. That is to say, each video signal sample is converted into a code word comprising a predetermined number of bits. This results in a digital color video signal which has in practice a bit rate of approximately 120 Megabits per second.
A considerably more advantageous bit rate is obtained by the use of DPCM, in which the aim is to reduce the redundancy in the color video signal. To this end, in one method of DPCM, this video signal is first sampled with the Nyquist sampling frequency and a prediction value is generated for each video signal sample thus obtained. Instead of the video signal sample itself, the difference between the video signal sample and its prediction value is now quantized and encoded (see, for example, References 1 and 2). DPCM can be realized with comparatively simple equipment and furnishes good results as long as no greater reduction of the bit rate is required than a reduction by a factor of three (see, for example, Reference 3).
A completely different manner of redundancy reduction is obtained by the use of TC (picture) transform coding; (see, for example, the References 2, 4 and 5). For TC, the TV picture is, as it were, divided into a large number of rectangular subpictures and each subpicture is thereafter assumed to be the sum of a number of mutually orthogonal basic pictures B(0), B(1), -, B(N-1), each having its own weighting factor y(0), y(1), -, y(N-1). Hereinafter these weighting factors will be denoted coefficients, as is customary, and it is these coefficients which are quantized and encoded.
In practice, to determine these coefficients, the video signal is first sampled at the Nyquist sampling frequency. The signal samples x(n) thus obtained may be used directly, or after analog-to-digital conversion, for further processing. The subpicture is now formed by N of these video signal samples, which all belong to either the same line signal or to different line signals. When each video signal sample x(n) of this subpicture is multiplied by a constant factor h(m,n) and the products obtained are added together, the coefficient y(m) is obtained. Mathematically, this operation can be expressed as follows: ##EQU1## The constant factors h(m,n) may be assumed to be the elements of an N.times.N-matrix H, which will be denoted a transformation matrix.
For black and white TV, where the video signal represents only one time-variable quantity, namely the brightness, the basic picture B(0) represents the average brightness of the subpicture and y(0) the amplitude value thereof. This coefficient is consequently the most important coefficient and must therefore be encoded with high accuracy. The remaining basic pictures B(1), -, B(N-1) furnish information about the detail in the subpicture. It appears that the coefficients y(1), -y(N-1) associated with these basic pictures may usually be encoded with considerably less accuracy. In practice the coefficient y(0) is generally converted into a code word z(0) having eight or nine bits, while each of the remaining coefficients y(m) is converted into a code word z(m) which comprises only 0, 1, 2, 3, 4 or 5 bits. As almost all the code words z(m) obtained in this way have a word length which is less than the word length of the PCM coded video signal samples x(n), the bit rate is also lower. By a suitable choice of the transformation matrix H, it is even possible to reduce this bit rate to a value below the bit rate obtained by DPCM coding of the video signal samples. The transformation matrices which are most frequently used in this connection are the Hotelling, the Fourier, the Hadamard and the Haar matrices.
Although in black and white TV a further reduction of the bit rate with respect to the DPCM can be obtained by the use of transform coding, the use of transform coding in the case of color TV results in hardly any reduction in bit rate, even compared to PCM. For color TV, DPCM coding of the video signal samples should be preferred. Reference 6 proves that in that case an additional reduction of the bit rate can be realized. In this Reference 6, it is proven that the PAL color video signal can be sampled with a sampling frequency f.sub.s which is twice as high as the color subcarrier frequency f.sub.sc, provided the sampling instants coincide only with the 45.degree. and 225.degree. phase positions of the color information signal u(t). Reference 7 describes that also the NTSC color video signal may be sampled with a sampling frequency which is twice as high as the color subcarrier frequency f.sub.sc, provided the sampling instants alternately coincide first during two line signals with the 45.degree. and 225.degree. phase positions and during two subsequent line signals with the 135.degree. and 315.degree. phase positions of the color information signal u(t).